Is there a role for naturally occurring cyanobacterial toxins in neurodegeneration?: The beta-N-methylamino-l-alanine (BMAA) paradigm

The naturally occurring, non-essential amino acid beta-N-methylamino-L-alanine (BMAA) has been recently found in high concentrations in brain tissues of patients with tauopathies such as the Amyotrophic Lateral Sclerosis-Parkinsonism-Dementia Complex (ALS/PDC) in the South Pacific island of Guam and in a small number of Caucasian, North American patients with sporadic Alzheimer's disease. BMAA is produced by cyanobacteria that are present in all conceivable aquatic and/or terrestrial ecosystems and may be accumulated in living tissues in free and protein-bound forms through the process of biomagnification. Although its role in human degenerative disease is highly debated, there is mounting evidence in support of the neurotoxic properties of BMAA that may be mediated via mechanisms involving among others the regulation of glutamate. Glutamate-related excitotoxicity is among the most prominent factors in the etiopathogenesis of human neurodegenerative diseases. Due to the wide geographical distribution of cyanobacteria and the possible implications of BMAA neurotoxic properties in public health more research towards this direction is warranted.     

Is there a role for naturally occurring cyanobacterial toxins in neurodegeneration?: The beta-N-methylamino-l-alanine (BMAA) paradigm

The naturally occurring, non-essential amino acid beta-N-methylamino-l-alanine (BMAA) has been recently found in high concentrations in brain tissues of patients with tauopathies such as the Amyotrophic Lateral Sclerosis-Parkinsonism-Dementia Complex (ALS/PDC) in the South Pacific island of Guam and in a small number of Caucasian, North American patients with sporadic Alzheimer's disease. BMAA is produced by cyanobacteria that are present in all conceivable aquatic and/or terrestrial ecosystems and may be accumulated in living tissues in free and protein-bound forms through the process of biomagnification. Although its role in human degenerative disease is highly debated, there is mounting evidence in support of the neurotoxic properties of BMAA that may be mediated via mechanisms involving among others the regulation of glutamate. Glutamate-related excitotoxicity is among the most prominent factors in the etiopathogenesis of human neurodegenerative diseases. Due to the wide geographical distribution of cyanobacteria and the possible implications of BMAA neurotoxic properties in public health more research towards this direction is warranted.     

Neurotoxic non-proteinogenic amino acid β-<Emphasis Type="Italic">N</Emphasis>-methylamino-L-alanine and its role in biological systems

Secondary metabolites of photoautotrophic organisms have attracted considerable interest in recent years. In particular, molecules of non-proteinogenic amino acids participating in various physiological processes and capable of producing adverse ecological effects have been actively investigated. For example, the non-proteinogenic amino acid β-Nmethylamino-L-alanine (BMAA) is neurotoxic to animals including humans. It is known that BMAA accumulation via the food chain can lead to development of neurodegenerative diseases in humans such as Alzheimer’s and Parkinson’s diseases as well as amyotrophic lateral sclerosis. Moreover, BMAA can be mistakenly incorporated into a protein molecule instead of serine. Natural sources of BMAA and methods for its detection are discussed in this review, as well as the role of BMAA in metabolism of its producers and possible mechanisms of toxicity of this amino acid in different living organisms.     

Valproate doubles the anoxic survival time of normal developing mice: possible relevance to valproate-induced decreases in cerebral levels of glutamate and aspartate, and increases in taurine

We have previously reported that chronic administration of valproate in developing mice decreased brain aspartic and glutamic acid levels and increased the brain taurine content. The direction of the valproate-induced changes in the cerebral levels of these neurotransmitter amino acids - excitatory in the case of aspartate and glutamate, inhibitory in the case of taurine - appeared relevant to the mechanism of its anticonvulsant action. Since the neuropathology of hypoxia-ischemia also appears to be mediated by release of glutamate/aspartate at the synapse, the valproate-induced reduction of the levels of these neuroexcitatory/neurotoxic amino acids suggested that valproate might increase the tolerance of young mice to anoxia. A doubling of the length of survival of the intact animal in an atmosphere of pure nitrogen gas and a three-fold increase in the duration of respiratory activity (gasping) of the isolated head after chronic administration of valproate support the speculation.     

The neurotoxic actions of quinolinic acid in the central nervous system

Excitotoxins such as kainic acid, ibotenic acid, and quinolinic acid are a group of molecules structurally related to glutamate or aspartate. They are capable of exciting neurons and producing axon sparing neuronal degeneration. Quinolinic acid (QUIN), an endogenous metabolite of the amino acid, tryptophan, has been detected in brain and its concentration increases with age. The content of QUIN in the brain and the activity of the enzymes involved in its synthesis and metabolism show a regional distribution. The neuroexcitatory action of QUIN is antagonized by magnesium (Mg2+) and the aminophosphonates, proposed N-methyl-D-aspartate (NMDA) receptor antagonists, suggesting that QUIN acts at the Mg2+ -sensitive NMDA receptor. Like its excitatory effects, QUIN's neurotoxic actions in the striatum are antagonized by the aminophosphonates. This suggests that QUIN neurotoxicity involves the NMDA receptor and (or) another receptor sensitive to the aminophosphonates. The neuroexcitatory and neurotoxic effects of QUIN are antagonized by kynurenic acid (KYN), another metabolite of tryptophan. QUIN toxicity is dependent on excitatory amino acid afferents and shows a regional variation in the brain. Local injection of QUIN into the nucleus basalis magnocellularis (NBM) results in a dose-dependent reduction in cortical cholinergic markers including the evoked release of acetylcholine. A significant reduction in cortical cholinergic function is maintained over a 3-month period. Coinjection of an equimolar ratio of QUIN and KYN into the NBM results in complete protection against QUIN-induced neurodegeneration and decreases in cortical cholinergic markers. In contrast, focal injections of QUIN into the frontoparietal cortex do not alter cortical cholinergic function.(ABSTRACT TRUNCATED AT 250 WORDS)     

Facilitated Transport of the Neurotoxin, β-N-Methylamino-l-Alanine, Across the Blood-Brain Barrier

beta-N-Methylamino-L-alanine (BMAA) is a neurotoxic plant amino acid that has been implicated in the pathogenesis of the high incidence amyotrophic lateral sclerosis and related parkinsonism dementia of the western Pacific. Previous studies have demonstrated that BMAA is taken up into brain following intravenous or oral administration. To examine the kinetics and mechanism of brain transfer, BMAA influx across the blood-brain barrier was measured in rats using an in situ brain perfusion technique. BMAA influx was found to be saturable with a maximal transfer rate (Vmax) of 1.6 +/- 0.3 x 10(-3) mumol/s/g and a half-saturation constant (Km) of 2.9 +/- 0.7 mM based on total perfusate BMAA concentration. Uptake was sodium independent and inhibitable by excess L-leucine, but not by L-lysine, L-glutamate, or methylaminoisobutyric acid, indicative of transfer by the cerebrovascular large neutral amino acid carrier. L-BMAA competitively reduced brain influx of L-[14C]leucine, as expected for cross-inhibition. The results demonstrate that BMAA is taken up into brain by the large neutral amino acid carrier of the blood-brain barrier and suggest that uptake may be sensitive to the same factors that affect neutral amino acid transport, such as diet, metabolism, disease, and age.     

β-Methylamino-L-alanine-induced protein aggregation in vitro and protection by L-serine

The cyanobacterial non-protein amino acid α-amino-β-methylaminopropionic acid, more commonly known as BMAA, was first discovered in the seeds of the ancient gymnosperm Cycad circinalis (now Cycas micronesica Hill). BMAA was linked to the high incidence of neurological disorders on the island of Guam first reported in the 1950s. BMAA still attracts interest as a possible causative factor in amyotrophic lateral sclerosis (ALS) following the identification of ALS disease clusters associated with living in proximity to lakes with regular cyanobacterial blooms. Since its discovery, BMAA toxicity has been the subject of many in vivo and in vitro studies. A number of mechanisms of toxicity have been proposed including an agonist effect at glutamate receptors, competition with cysteine for transport system x_c^_ and other mechanisms capable of generating cellular oxidative stress. In addition, a wide range of studies have reported effects related to disturbances in proteostasis including endoplasmic reticulum stress and activation of the unfolded protein response. In the present studies we examine the effects of BMAA on the ubiquitin-proteasome system (UPS) and on chaperone-mediated autophagy (CMA) by measuring levels of ubiquitinated proteins and lamp2a protein levels in a differentiated neuronal cell line exposed to BMAA. The BMAA induced increases in oxidised proteins and the increase in CMA activity reported could be prevented by co-administration of L-serine but not by the two antioxidants examined. These data provide further evidence of a protective role for L-serine against the deleterious effects of BMAA.

     

Co-occurrence of beta-N-methylamino-L-alanine, a neurotoxic amino acid with other cyanobacterial toxins in British waterbodies, 1990-2004

The neurotoxic amino acid, β- N -methylamino- l -alanine, was found to be present in all of 12 analysed samples of cyanobacterial blooms, scums and mats, which had been collected in seven years between 1990 and 2004 inclusive and stored at −20°C. BMAA identification was by high performance liquid chromatography with fluorescence detection and by triple quadrapole mass spectrometry after derivatization. The samples originated from 11 freshwater lakes and 1 brackish waterbody, used either for drinking water, recreation, or both. BMAA was present at between 8 and 287 μg g⁻¹ cyanobacterial dry weight and was present as both the free amino acid and associated with precipitated proteins. Ten of the samples contained additional cyanotoxins (including microcystins, anatoxin-a, nodularin and saxitoxin) at the time of sample collection. Five of the samples were associated with animal deaths, attributable at the time of sample collection, to microcystins, nodularin or anatoxin-a. The data demonstrate the presence of BMAA by high performance liquid chromatography and mass spectrometry in a diverse range of cyanobacterial bloom samples from high resource waterbodies. Furthermore, samples collected over several years shows that BMAA can co-occur with other known cyanotoxins in such waterbodies. Health risk assessment of cyanobacterial BMAA in waterbodies is suggested.     

DL-7-azatryptophan and citrulline metabolism in the cyanobacterium Anabaena sp. strain 1F.

An alternative route for the primary assimilation of ammonia proceeds via glutamine synthetase-carbamyl phosphate synthetase and its inherent glutaminase activity in Anabaena sp. strain 1F, a marine filamentous, heterocystous cyanobacterium. Evidence for the presence of this possible alternative route to glutamate was provided by the use of amino acid analogs as specific enzyme inhibitors, enzymological studies, and radioistopic labeling experiments. The amino acid pool patterns of continuous cultures of Anabaena sp. strain 1F were markedly influenced by the nitrogen source. A relatively high concentration of glutamate was maintained in the amino acid pools of all cultures irrespective of the nitrogen source, reflecting the central role of glutamate in nitrogen metabolism. The addition of 1.0 microM azaserine increased the intracellular pools of glutamate and glutamine. All attempts to detect any enzymatic activity for glutamate synthase by measuring the formation of L-[14C]glutamate from 2-keto-[1-14C]glutarate and glutamine failed. The addition of 10 microM DL-7-azatryptophan caused a transient accumulation of intracellular citrulline and alanine which was not affected by the presence of chloramphenicol. The in vitro activity of carbamyl phosphate synthetase and glutaminase increased severalfold in the presence of azatryptophan. Results from radioisotopic labeling experiments with [14C]bicarbonate and L-[1-14C]ornithine also indicated that citrulline was formed via carbamyl phosphate synthetase and ornithine transcarbamylase. In addition to its effects on nitrogen metabolism, azatryptophan also affected carbon metabolism by inhibiting photosynthetic carbon assimilation and photosynthetic oxygen evolution.     

Differential excitatory effects of kainic acid on CA3 and CA1 hippocampal pyramidal neurons: Further evidence for the excitotoxic hypothesis and for a receptor-mediated action

CA₃ hippocampal pyramidal neurons are known to be extremely susceptible to the neurotoxic action of kainic acid (KA). The excitotoxic hypothesis claims that cytotoxic amino acids exert this effect via neuroexcitation. To put this hypothesis to the test, the responsiveness of CA₃ neurons to KA was assessed by means of microiontophoresis and compared to that of CA₁ and cortical neurons. CA₃ pyramidal neurons showed an extreme sensitivity to KA, much greater than that of CA₁ and cortical neurons. There was no such differential responsiveness to neither acetylcholine nor glutamate (GLU). The exquisite sensitivity of CA₃ neurons to both neurotoxic and neuroexcitatory actions of KA supports the excitotoxic hypothesis. The clear dissociation between the effects of KA and GLU on hippocampal pyramidal cells indicates that KA-induced activation is not mediated by GLU receptors and favors the notion that KA might act on specific receptors.     

Arabidopsis mutants resistant to S(+)-beta-methyl-alpha, beta-diaminopropionic acid, a cycad-derived glutamate receptor agonist

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that are the predominant neuroreceptors in the mammalian brain. Genes with high sequence similarity to animal iGluRs have been identified in Arabidopsis. To understand the role of Arabidopsis glutamate receptor-like (AtGLR) genes in plants, we have taken a pharmacological approach by examining the effects of BMAA [S(+)-beta-methyl-alpha, beta-diaminopropionic acid], a cycad-derived iGluR agonist, on Arabidopsis morphogenesis. When applied to Arabidopsis seedlings, BMAA caused a 2- to 3-fold increase in hypocotyl elongation and inhibited cotyledon opening during early seedling development. The effect of BMAA on hypocotyl elongation is light specific. Furthermore, BMAA effects on early morphogenesis of Arabidopsis can be reversed by the simultaneous application of glutamate, the native iGluR agonist in animals. To determine the targets of BMAA action in Arabidopsis, a genetic screen was devised to isolate Arabidopsis mutants with a BMAA insensitive morphology (bim). When grown in the light on BMAA, bim mutants exhibited short hypocotyls compared with wild type. bim mutants were grouped into three classes based on their morphology when grown in the dark in the absence of BMAA. Class-I bim mutants have a normal, etiolated morphology, similar to wild-type plants. Class-II bim mutants have shorter hypocotyls and closed cotyledons when grown in the dark. Class-III bim mutants have short hypocotyls and open cotyledons when grown in the dark, resembling the previously characterized constitutively photomorphogenic mutants (cop, det, fus, and shy). Further analysis of the bim mutants should help define whether plant-derived iGluR agonists target glutamate receptor signaling pathways in plants.     

Molecular orbital calculation for the model compounds of kainoid amino acids, agonists of excitatory amino acid receptors. Does the kainoid C4-substituent directly interact with the receptors?

Kainoid amino acids are agonists of the AMPA/kainate receptors and exhibit highly potent neuroexcitatory activity. From the results of extensive structure--activity relationship studies, we previously postulated that the C4-substituent of the kainoid amino acids interacts with an allosteric site of the glutamate receptor with electron-donating character. In order to investigate the mode of action in more detail, molecular orbital calculation for model compounds of the kainoid were performed. The results indicated that the HOMO energy level of the C4-substituent is involved in the potent neuroexcitatory activity, thus supporting our hypothesis.     

Effects of ketone bodies on amino acid metabolism in isolated rat diaphragm.

1. Diaphragms from 48h-starved rats were incubated in Krebs-Ringer bicarbonate medium at 37degreesC for 30min and then transferred into new medium and incubated for 1, 2 and 3 h. 2. The amount of free amino acids found at the end of each time of incubation was larger than the amount at the beginning of incubation, indicating that in this system proteolysis is prevailing. 3. The diaphragms was releasing mainly alanine and glutamine into the incubation medium. 4. Within the periods of incubation the release and metabolism of free amino acids was proceeding at a constant rate. 5. Addition of sodium DL-3-hydroxybutyrate decreased the tissue content of several amino acids, among which were tyrosine and phenylalanine, suggesting that proteolysis was decreased by ketone bodies. 6. In the presence of glucose (10mM) and branched-chain amino acids (0.5mM), sodium DL-3-hydroxybutyrate at concentrations of 4 or 6 mM resulted in 30% decrease in tissue alanine content and a 20% decline in alanine release. Release of taurine and glutamine was decreased by 19 and 16% respectively with 6 mM-sodium DL-3-hydroxybutyrate. Addition of sodium acetoacetate (1-3mM) also resulted in a 20-35% decrease in tissue content of alanine, glutamine and taurine and in a 15-24% decrease of alanine and glutamine release. Smaller decreases (less than 15%) in the release of glycine, threonine, proline, serine and aspartate were also observed in the presence of sodium DL-3-hydroxybutyrate or sodium acetoacetate. 7. Substitution of pyruvate (1.0mM) for glucose in the presence of acetoacetate restored alanine and glutamine production to control values. In the presence of acetoacetate, pyruvate also increased the tissue content of aspartate by 77% and decreased the tissue content of glutamate by 30%. 8. It is suggested that in diaphragms from starved rats, ketone bodies (a) in the absence of other substrates inhibit protein catabolism and (b) in the presence of glucose and branched-chain amino acids decrease alanine and glutamine production, by inhibiting glycolysis.     

The non-protein amino acid β-N-methylamino-l-alanine in Portuguese cyanobacterial isolates

The tailor made amino acid β-N-methyl-amino-L-alanine (BMAA) is a neurotoxin produced by cyanobacteria. It has been associated with certain forms of progressive neurodegenerative disease, including sporadic Amyotrophic Lateral Sclerosis and Alzheimer's disease. Some different reports of BMAA in cyanobacterial blooms from lakes, reservoirs, and other water resources have been made by different investigators. We here report the detection of BMAA of both free and protein-bound produced by cyanobacteria, belonging to the Chroococcales, Oscillatoriales and Nostocales ordered. We use a rapid and sensitive HPLC-FD method that utilizes methanol elution and the Waters AQC Tag chemistry. On other hand, we have used three different assay procedures for BMAA extraction from cyanobacteria: Trichloroacetic acid (TCA), Methanol/Acetone and hydrochloric acid (HCl). All assays let successfully detect BMAA in all cyanobacteria samples analyzed. Nevertheless, with TCA and HCl extraction procedures the highest BMAA values, for free as well as protein-bound BMAA were detected. BMAA content could not be related to the taxonomy of the isolates or to their geographical origin, and no correlation between free and protein-bound BMAA concentrations were observed within or between taxonomic groups. These data offer confirmation of the taxonomic and geographic ubiquity of BMAA from naturally occurring populations of cyanobacteria, for the first time reported for estuaries.     

The natural non-protein amino acid N-β-methylamino-l-alanine (BMAA) is incorporated into protein during synthesis

N-β-methylamino-l-alanine (BMAA) is an amino acid produced by cyanobacteria and accumulated through trophic levels in the environment and natural food webs. Human exposure to BMAA has been linked to progressive neurodegenerative diseases, potentially due to incorporation of BMAA into protein. The insertion of BMAA and other non-protein amino acids into proteins may trigger protein misfunction, misfolding and/or aggregation. However, the specific mechanism by which BMAA is associated with proteins remained unidentified. Such studies are challenging because of the complexity of biological systems and samples. A cell-free in vitro protein synthesis system offers an excellent approach for investigation of changing amino acid composition in protein. In this study, we report that BMAA incorporates into protein as an error in synthesis when a template DNA sequence is used. Bicinchoninic acid assay of total protein synthesis determined that BMAA effectively substituted for alanine and serine in protein product. LC–MS/MS confirmed that BMAA was selectively inserted into proteins in place of other amino acids, but isomers N-(2-aminoethyl)glycine (AEG) and 2,4-diaminobutyric acid (DAB) did not share this characteristic. Incorporation of BMAA into proteins was significantly higher when genomic DNA from post-mortem brain was the template. About half of BMAA in the synthetic proteins was released with denaturation with sodium dodecylsulfonate and dithiothreitol, but the remaining BMAA could only be released by acid hydrolysis. Together these data demonstrate that BMAA is incorporated into the amino acid backbone of proteins during synthesis and also associated with proteins through non-covalent bonding.     

Utilization of the Synthetic Phosphagen Cyclocreatine Phosphate by a Simple Brain Model During Stimulation by Neuroexcitatory Amino Acids

The ability of 1-carboxymethyl-2-imino-3-phosphonoimidazolidine (cyclocreatine-P), accumulated by a simple brain model, to function as a supplemental synthetic phosphagen and respond to the decreases in cytosolic ATP/free ADP ratios that occur during prolonged stimulation by various excitatory amino acids was investigated. Suspensions of chopped whole brain from 11- to 14-day-old chick embryos were incubated with 30 mM cyclocreatine for 90 min, resulting in accumulation of 100 mumol/g dry weight of cyclocreatine-P, and then incubated for up to 1 h with a series of excitatory amino acids of widely differing potencies. Under these conditions net utilization of cyclocreatine-P was detected in response to stimulation by the following neuroexcitatory compounds at the indicated threshold concentrations: kainate (20 microM), N-methyl-DL-aspartate (20 microM), L-homocysteate (20 microM), L-glutamate (200 microM), D-glutamate (200 microM), L-aspartate (2 mM), DL-2-amino-3-phosphonopropionate (2 mM), and DL-2-amino-4-phosphonobutyrate (2 mM). Significant increases in water content of chick embryo brain minces accompanied stimulation by excitatory amino acids. It is suggested that changes in water content or cyclocreatine-P levels in this sensitive brain model might be utilized in automatable screening procedures for detecting novel antagonists and/or new agonists of excitatory amino acids.     

Distribution of the neurotoxic nonprotein amino acid BMAA in Cycas micronesica

BMAA (β-methylamino-L-alanine), a nonprotein amino acid with neurotoxic properties occurs in the cycad Cycas micronesica Hill in Guam. BMAA may have originally played a role as an antiherbivory compound in the plant, but is now of great interest because of its possible link to ALS-PDC, a neurological disease among the Chamorro people of Guam. Biomagnified cycad neurotoxins may play a role because they accumulate in flying foxes of the genus Pteropus that are eaten during traditional feasts. Since flying foxes feed on the seed sarcotesta it is important to understand the distribution of BMAA in the various tissues of the cycad. Using HPLC techniques, we quantified both BMAA and glutamic acid (GLU) in all cycad tissues. Although GLU is distributed randomly throughout the plant, BMAA is concentrated in cycad reproductive organs, with the highest concentrations being found in the immature staminate sporangium and the outmost layer of the sarcotesta. This finding is consistent with the putative evolutionary role of BMAA as an antiherbivory compound, as well as the biomagnification of the compound in flying foxes that ingest the seed sarcotesta.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 165–168.     

Retention of the cyanobacterial neurotoxin β‐N‐methylamino‐l‐alanine in melanin and neuromelanin‐containing cells – a possible link between Parkinson‐dementia complex and pigmentary retinopathy

β‐N‐methylamino‐l ‐alanine (BMAA), a neurotoxic amino acid produced by cyanobacteria, has been suggested to be involved in the etiology of a neurodegenerative disease complex which includes Parkinson‐dementia complex (PDC). In PDC, neuromelanin‐containing neurons in substantia nigra are degenerated. Many PDC patients also have an uncommon pigmentary retinopathy. The aim of this study was to investigate the distribution of ³H‐BMAA in mice and frogs, with emphasis on pigment‐containing tissues. Using autoradiography, a distinct retention of ³H‐BMAA was observed in melanin‐containing tissues such as the eye and neuromelanin‐containing neurons in frog brain. Analysis of the binding of ³H‐BMAA to Sepia melanin in vitro demonstrated two apparent binding sites. In vitro‐studies with synthetic melanin revealed a stronger interaction of ³H‐BMAA with melanin during synthesis than the binding to preformed melanin. Long‐term exposure to BMAA may lead to bioaccumulation in melanin‐ and neuromelanin‐containing cells causing high intracellular levels, and potentially changed melanin characteristics via incorporation of BMAA into the melanin polymer. Interaction of BMAA with melanin may be a possible link between PDC and pigmentary retinopathy.     

The physiological effect of ingested β-N-methylamino-L-alanine on a glutamatergic synapse in an in vivo preparation

The neurotoxin, BMAA (β-N-methylamino-L-alanine), may be a risk factor for amyotrophic lateral sclerosis (ALS), Parkinson's (PD) and Alzheimer's (AD) disease. In vivo experiments have demonstrated that BMAA can cause a number of motor dysfunctions if ingested or injected, and in vitro experiments show that this toxin binds to glutamate receptors with deleterious results. Also, BMAA exists in the human food chain worldwide, and has been detected in the brains of ALS and AD patients. This paper offers the first demonstration by intracellular recording of the effect of ingested BMAA on the postsynaptic response of an identified glutamatergic cell in a living, undissected organism (Drosophila melanogaster), and correlates these observations with the specific motor dysfunctions that result from ingestion. The results suggest that BMAA acts as a glutamate agonist, causing NMDA receptor channels to remain open for prolonged periods of time, thereby damaging the cell by excitotoxicity. The effect on the postsynaptic response became apparent days before the function of the postsynaptic cell (wing beat) became affected. Severely depolarized cells were able to fully recover with the removal of BMAA from the food source, suggesting that blocking BMAA binding in the brain might be a good treatment strategy.     

L-glutamate stimulation of Na+ efflux from brain synaptic membrane vesicles

The characteristics of 22Na efflux from 22NaCl-preloaded synaptic plasma membrane vesicles and the stimulation of such efflux by gramicidin D and L-glutamate were determined. The rate and magnitude of passive Na+ efflux were dependent on the initial intravesicular NaCl concentration. A Na+:cation exchange process was also observed. Gramicidin D markedly enhanced Na+ efflux in a concentration-dependent manner and at 10 microM it caused total loss of intravesicular 22Na. The neuroexcitatory amino acids L-glutamate and D-glutamate, and the amino acid analog kainic acid, also stimulated Na+ efflux in a dose-dependent fashion, but their effects were weaker than those of gramicidin D. The mechanism of glutamate stimulation of Na+ flux is presumed to be through the activation of the glutamate receptor . Na+ channel complex in these membranes.